Oxides of manganese are utilized for a number of industrial applications, such as pollution control systems, steel manufacture, batteries and catalytic converters, to name a few. Of particular, but not exclusive, interest to Applicants is the use of oxides of manganese in pollution control systems. Applicants are co-inventors of the subject matter of co-pending U.S. patent applications Ser. No. 09/919,600, now U.S. Pat. No. 6,610,263, issued Aug. 26, 2003, Ser. No. 09/951,697, now abandoned, Ser. No. 10/044,089, now U.S. Pat. No. 6,579,507, issued Jun. 17, 2003 and Ser. No. 10/025,270, filed Oct. 18, 2001, the disclosures of which are incorporated herein by reference. These applications disclose pollutant removal systems and processes, known as Pahlman™ systems and processes, which utilize dry and wet removal techniques and combinations thereof incorporating the use of oxides of manganese as a sorbent for capture and removal of target pollutants from gas streams.
The term “target pollutant,” as used herein, refers to the pollutant or pollutants that are to be captured and removed from a gas stream. Examples of target pollutants that may be removed with an oxide of manganese sorbent include, but are not limited to, NOX, SOX, mercury (Hg) and mercury compounds, H2S and other totally reduced sulfides (TRS), chlorides, such as hydrochloric acid (HCl), and oxides of carbon (CO and CO2).
Before going further, the following additional definitions will be with respect to this background discussion and to the understanding of the invention disclosed herein:
“Reacted” or “loaded,” as used interchangeably herein, refers in conjunction with “oxides of manganese” and/or “sorbent” to oxides of manganese or sorbent that has interacted with one or more target pollutants in a gas whether by chemical reaction, adsorption or absorption. The term does not mean that all reactive or active sites of the sorbent have been utilized as all such sites may not actually be utilized.
“Unreacted” or “virgin,” as used interchangeably herein, refers in conjunction with “oxides of manganese” and/or “sorbent” to oxides of manganese or sorbent that has not interacted with target pollutants in a gas or gas stream.
“Nitrates of manganese,” as used herein, refers to and includes the various forms of manganese nitrate, regardless of chemical formula, that may be formed through the chemical reaction between NOX and the sorbent and includes hydrated forms as well.
“Sulfates of manganese,” as used herein, refers to and includes the various forms of manganese sulfate, regardless of chemical formula that may be formed through the chemical reaction between SOX and the sorbent and includes hydrated forms as well.
Oxides of manganese in various forms, utilized as sorbents, are introduced into the Pahlman™ systems (and other pollution removal systems) and interact with the target pollutants in gas streams routed through the systems as a catalyst, a reactant, an absorbent or an adsorbent. During such interaction in the process of pollutant removal, the oxidation (or valence) state of the oxides of manganese sorbent is reduced from its original state during reaction with the target pollutants. For example, where the target pollutants are NOX or SO2, pollutant removal occurs possibly through overall reactions such as the following:SO2+MnO2→MnSO4  Reaction (1)2NO+O2+MnO2→Mn(NO3)2  Reaction (2)
In both of the reactions above, manganese (Mn) is reduced from the +4 valence state to +2 valence state during formation of the reaction products shown. It should be noted that the actual reactions may include other steps not shown, and that indicating Reactions 1 and 2 is solely for illustrative purposes.
The element manganese (Mn), and therefore oxides of manganese, may exist in six different valence (oxidation) states. Of particular interest and usefulness for gaseous pollutant removal are those oxides of manganese having valence states of +2, +3, and +4, which correspond to the oxides MnO, Mn2O3, MnO2 and Mn3O4. The oxide Mn3O4 is believed to be a solid-solution of both the +2 and +3 states.
A characteristic of most oxides of manganese species is non-stoichiometry. For example; most MnO2 species typically contain on average less than the theoretical number of 2 oxygen atoms, with numbers more typically ranging from 1.5 to 2.0. The non-stoichiometry characteristic of oxides of manganese is thought to result from solid-solution mixtures of two or more oxide species (such as may occur in the oxide Mn3O4), or distortions of molecular structure and exists in all but the beta (β), or pyrolusite, form of manganese dioxide. Oxides of manganese having the formula MnOX where X is about 1.5 to about 2.0 are particularly suitable for use as sorbent for dry removal of target pollutants from gas streams and may be also be utilized in wet removal. However, the most active types of oxides of manganese for use as a sorbent for target pollutant removal usually have the formula MnO1.7 to 1.95, which translates into average manganese valence states of +3.4 to +3.9, as opposed to the theoretical +4.0 state. It is unusual for average valence states above about 3.9 to exist in most forms of oxides of manganese.
Oxides of manganese are known to exhibit several identifiable crystal structures, which result from different assembly combinations of their basic molecular structural units. These basic structural “building block” units are MnO6 octahedra, which consist of one manganese atom at the geometric center, and one oxygen atom at each of the six apex positions of an octahedral geometrical shape. The octahedra may be joined together along their edges and/or corners, to form “chain” patterns, with void spaces (“tunnels”). Regular (and sometimes irregular) three-dimensional patterns consist of layers of such “chains” and “tunnels” of joined octahedra. These crystalline geometries are identified by characteristic x-ray diffraction (XRD) patterns. Most oxides of manganese are classifiable into one or more of the six fundamental crystal structures, which are called alpha (α), beta (β), gamma (γ), delta (δ), epsilon (ε), and ramsdellite. Certain older literature also included rho (ρ) and lambda (λ) structures, which are now thought obsolete, due partly to improvements in XRD technique. Some (amorphous) forms of MnO2 exhibit no crystalline structure.
Certain characteristics of oxides of manganese probably arise from the size and shape of voids within these crystalline patterns and from certain elements, and compounds, which may occupy the voids and appear to help prevent collapse of certain structures. Applicants believe that these characteristics in addition to the oxidation state may have an affect upon the loading capacity of oxides of manganese sorbent. Further, many oxides of manganese, including those that are the subject of the present, come in hydrated or hydrous forms, having water chemically bound or combined to or within their crystalline structures, containing one or more molecules of water; this is sometimes referred to as bound water, structural water, water of crystallization or water of hydration. In these forms, the water is combined is such a way that it may be removed with sufficient heat without substantially changing the chemical structure of the oxides of manganese. Such oxides of manganese are also useful as a sorbent. This bound water may also contribute to the chemical reactivity and possibly catalytic behavior of the species.
Some oxides of manganese have the ability to absorb oxygen from gas. Manganous oxide (MnO) and Mn(OH)2 will oxidize to MnO2 in the presence of air, for example. Additionally, the dioxides of manganese are themselves oxidizers. They readily exchange oxygen in chemical reactions and are known to have catalytic properties. This oxygen exchange ability may be related to proton mobility and lattice defects common within most MnO2 crystal structures.
The oxidizing potential of MnO2 is advantageously utilized in target pollutant removal in the Pahlman™ and other pollutant removals systems and processes. Target pollutants, such as NOX, SO2, CO, and CO2 gases, mercury (Hg) and other pollutants, require oxidation of the species prior to reaction with MnO2 sorbent to form reaction products, such as manganese sulfates, nitrates, and carbonates, mercury compounds, and other corresponding reaction products, in order for them to be captured and removed from gas streams.
Manganese compounds or salts are soluble in water in the +2 valence state, but not in the +4 state. Manganese compounds having an average valence state approaching +2 are soluble in water, while those with average valence states approaching +4 are not. Therefore Mn+2 compounds, including MnO are readily soluble in aqueous solutions, as opposed to MnO2. During the formation of reaction products such as manganese nitrates and sulfates, the manganese is reduced from about the +4 state to the +2 state. This property allows the reaction products formed on the surface of oxides of manganese sorbent particles to be readily dissolved and removed from the sorbent particles in aqueous solutions by disassociation into reaction product anions, such as sulfate or, nitrate, and manganese cations such as Mn+2 cations.
Manganese dioxides are divided into three origin-based categories, which are: 1) natural (mineral) manganese dioxide (NMD), 2) chemical manganese dioxide (CMD), and 3) electrolytic manganese dioxide (EMD). As implied, NMD occurs naturally as various minerals, which may be purified by mechanical or chemical means. The most common form of NMD is pyrolusite (β-MnO2), which is inexpensive, but has rather low chemical activity and therefore low pollutant loading capacity. CMD and EMD varieties are synthetic oxides of manganese. EMD is produced primarily for the battery industry, which requires relatively high bulk density (which often results from relatively large, compact particles), relatively high purity, and good electrochemical activity. Though useful as sorbent, characteristics such as low surface area and large compact particle size make EMD somewhat inferior to CMD for gas removal applications, despite its good electrochemical activity. Chemically synthesized oxides of manganese of all kinds fall into the CMD category and includes chemically treated or pretreated oxides of manganese. In chemical synthesis, a great deal of control is possible over physical characteristics such as particle size and shape, porosity, composition, surface area, and bulk density in addition to electrochemical or oxidation potential. It is believed that these characteristics contribute to the loading capacity of some oxides of manganese.
Oxides of manganese have the ability to capture target pollutants from gas streams, however, the low pollutant loading rates achieved with various prior art oxides of manganese have made some industrial applications of this ability uneconomical. The low target pollutant loading rates of various prior art oxides of manganese sorbents would require voluminous amounts to effectively capture large quantities of target pollutants that exist at many industrial sites, e.g., NOx and/or SO2. The large quantity of sorbent that would be required to capture NOx and/or SO2 could result in an overly costly pollutant removal system and sorbent regeneration system. It would therefore be desirable to enhance the loading capacities of the oxides of manganese sorbent in order to economically implement a pollution removal system utilizing oxides of manganese.
It is believed that reaction products, such as the manganese salts of Reaction (1) and Reaction (2) above, form on the surfaces of the sorbent particles of oxides of manganese. These reactions may extend to some depth inside the sorbent particles and into the pores and micro fissures. Applicants believe that formation of such reactions products occurs primarily on the surfaces of the oxides of manganese particles, resulting in a layer or coating, which effectively isolates the covered portion of the particle surface and thereby prevents continued rapid reaction with additional target pollutants. Further, the oxidation state and thus the loading capacity of the oxides of manganese below the surface of the reaction product coating may be reduced during the pollutant removal, thus diminishing the loading capacity of sorbent even after the reaction product have been removed or disassociated into an aqueous solution. It would therefore be desirable for economic reasons to re-use or regenerate the unreacted portions of the sorbent for subsequent cycles of pollutant gas removal.
In order to regenerate the reacted oxides of manganese effectively for subsequent reuse as a gas sorbent with high removal efficiencies and target pollutant loading rates, it is advantageous to: (1) remove soluble reaction products or reaction product salts, such as salts MnSO4, Mn(NO3)2, MnCl2 and other manganese halides, manganese salt reaction products, and the like, from the sorbent particle surfaces with an aqueous solution through disassociation into their constituent cations and anions, e.g, Mn+2, Cl-1 SO4-2, and NO3-1 ions; (2) restore or increase the target pollutant loading capacity and/or oxidation state of the remaining solid oxides of manganese sorbent below the surface of the reaction product coating that is not dissociated in an aqueous solution, (3) recover, through precipitation, the Mn+2 ions that were dissociated into solution from the reaction products formed through reactions with the various target pollutants; and (4) to recover other ions and form marketable or otherwise useful by-products. Note that some soluble and insoluble reaction products may be removed through thermal decomposition.
Applicants have developed methods of producing newly precipitated oxides of manganese, of treating commercially available virgin oxides of manganese, and of regenerating loaded oxides of manganese that results in the production of oxides of manganese useful, amongst other applications as sorbent for pollutant removal. Oxides of manganese so produced may exhibit high or increased loading capacity and/or valence states as compared to reacted and virgin oxides of manganese of various forms, including a variety of commercially available oxides of manganese. Applicants have additionally developed a system and process for cyclically loading, with target pollutants, and regenerating oxides of manganese sorbent that results in the production of useful byproducts.